This invention relates generally to cryogenic cooling machines, and more particularly to a machine adapted to operate with cartridges containing pre-cooked meals to effect rapid cooling thereof.
My above-identified co-pending application discloses a fast food technique and apparatus therefor in which pre-cooked meals which have been kept under refrigeration are thereafter reheated in a hot air oven and made directly available to customers. The nature of the cooking, refrigeration and reheating operations are such that the essential texture, flavor and nutritional value of the food are maintained without significant degradation.
In this fast food technique, each cooked meal is placed in an individual tray and sealed, a stack of such trays being nested within an open carton to form a cartridge which facilitates subsequent handling and processing. The side walls of the carton are provided with holes to admit a heated or refrigerated gas which is circulated in the spaces between the trays in the stack to heat or cool the meals therein, as the case may be.
These cartridges, prior to being placed in an oven for reheating the pre-cooked meals, must be kept in a refrigerated storage chamber to maintain the food temperature just above the freezing point for as long as is necessary. In practice, this temperature may be in the range of about 20.degree. to 30.degree. F.; for when the moisture content of the food is rich in dissolved salts, the freezing point may be well below 32.degree. F. It is vital that the food not be refrigerated below the freezing point, for the formation of ice crystals in the food is destructive of food texture and must be avoided. Also, it is important that the trays be sealed to prevent the loss of moisture and volatile constituents.
In the large scale production of cooked meals in connection with a fast food technique of the type disclosed in my copending application, after the meals have been cooked and placed in sealed trays, it is essential that the temperature of the hot meals in the trays be reduced in temperature quickly to a level approaching the cold temperature of the refrigerated chamber in which the trays are to be stored; for one cannot, as a practical matter, abruptly load the storage chamber with a large quantity of hot food. To this end, a blast tunnel may be used, but this is relatively inefficient and costly to operate.
In a prior attempt to accelerate the action of a conventional mechanical refrigeration system and to overcome certain shortcomings thereof, the Overbye U.S. Pat. No. 3,115,756 discloses an arrangement in which the food to be cooled or frozen is conveyed through an open tunnel on a foraminous belt below which are disposed the evaporation coils of a mechanical refrigeration system. Air is blown upwardly through the coils and the resultant cold air is then forced through the belt to effect rapid cooling of the food advancing through the tunnel.
While the Overbye arrangement is more efficient than most blast tunnel freezers, it has serious drawbacks, among which is the formation of snow and ice on the refrigeration coils and other components of the system as a result of moisture extracted from the food being processed and from the ambient air. The formation of such snow and ice markedly reduces the thermal efficiency of the refrigeration system.
Another prior approach to rapid cooling is by means of cryogenic liquids such as liquid carbon dioxide, liquid nitrogen or liquid air having normal boiling points well below -100.degree. F. Thus in the patents to Macintosh, U.S. Pat. Nos. 3,238,138 and 3,255,608, there are disclosed cryogenic refrigeration machines in which food is conveyed on a belt through an open-ended tunnel in which the food is sprayed with ultra-cold liquid nitrogen whose temperature is -320.degree. F. to effect quick freezing thereof.
In the improved cryogenic freezing machine disclosed in the Hart U.S. Pat. No. 3,427,820, the food is conveyed through an elongated open-ended tunnel toward a freezing zone where the food is exposed to atomized liquid nitrogen. Pre-cooling of the food is effected by cold gas evolved in the freezing zone, the cold gas being sucked into a pre-cooling zone in advance of the freezing zone. In the Hart machine, rapid freezing is effected by the combined action of the latent heat of vaporization of the liquid nitrogen and the specific heat of the cold gas derived from the liquid. The heat energy necessary to transform the state of the liquid nitrogen into gas in the freezing zone is taken from the food exposed to atomized liquid nitrogen in this zone, whereas in the pre-freezing zone, the heat energy, or BTUs, which raises the temperature in the cold gas is absorbed from the advancing food which travels in heat exchange relationship with the cold gas.
In cryogenic freezing machines of the Hart and Macintosh type, the ends of the tunnel are open. No use is made of gas locks, for such locks complicate and slow down the conveyance of the food through the tunnel. Hence to minimize the entry of warm ambient air into the tunnel, the inlet and outlet is made as small as possible, consistent with the dimensions of the food to be processed in the machine. Also, in such prior machines, the food to be frozen cannot be sealed, for the food must be directly exposed to the liquid nitrogen.
These prior machines are therefore inappropriate to the fast food technique which is the concern of the present invention; for in this technique the pre-cooked meals must be sealed in trays to avoid the loss of moisture and volatile constituents, and freezing of the food as would occur with direct contact with liquid nitrogen must be avoided--for the desideratum in this technique is food cooled to a point just above freezing.